FIG. 1 shows one example of a wireless communication system. Referring to FIG. 1, a wireless communications system 100 consists of a plurality of base stations 110 and a plurality of mobile stations 120. The wireless communications system 100 can include a homogeneous network or a heterogeneous network. In this case, the heterogeneous network represents the network in which different network entities including macro cells, femto cells, relays and the like coexist. The base station is a fixed station that communicates with a mobile station in general. And, the base stations 110a, 110b and 110c provide services to specific geographical areas 102a, 102b and 102c, respectively. In order to enhance system performance, each of the specific geographical areas can be further divided into a plurality of smaller regions 104a, 104b and 104c for example. Each of the smaller areas can be named a cell, a sector or a segment. In IEEE 802.16e system, a cell identity is given with reference to a whole system. On the contrary, a sector or segment identity is given with reference to a specific area in which each base station provides a service and has a value set to one of 0 to 2. The mobile station 120 is distributed in the wireless communication system in general and may be fixed or movable. Each of the mobile stations is able to communicate with at least one base station at a random moment in uplink ((UL) or downlink (DL). A base station and a mobile station are able to communicate with each other using one of CDMA (code division multiple access) system, FDMA (frequency division multiple access) system, TDMA (time division multiple access) system, SC-FDMA (single carrier frequency division multiple access) system, MC-FDMA (multi carrier frequency division multiple access) system, OFDMA (orthogonal frequency division multiple access) system, a combination thereof and the like. In this specification, ‘uplink’ may indicate a communication link from a mobile station to a base station. And, ‘downlink’ may indicate a communication link from a base station to a mobile station.